Unplug the electrical connector to the resistor. There are 4 terminals on the resistor block. You test the resistor block using an electrical multi-meter, set to the Ohms scale. Using the meter's test leads, check the following:
Between terminals 2 & 3, you should have 2.3 ohms resistance.
Between terminals 3 &4 you should have 1.1 ohms resistance.
Between terminals 3 & 1 you should have 0.4 ohms resistance.
A bad resistor block will generally show infinite ohms on one or more of these tests. Look very carefully [magnifying glass] for the marks which identify the terminals on the block.

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Could be a number of thing's ! Testing the electrical circuit is the way to find out what's wrong . Using a wiring diagram and a DVOM - digital volt ohm meter . Videos on youtube showing how to test it ! Without testing i would only be guessing . Does it work in high speed ? In high speed it by-passes the resistor . If it doesn't work in any speed more then likely the blower motor is the problem !
Air Speed
The HVAC control assembly applies ground to the blower motor control circuit that corresponds to the selected blower speed. The resistors and the blower motor are in a series circuit. The following list represents the number of resistors in series with the blower motor per particular speed request:
• Low speed--3 resistors
• Medium 1 speed--2 resistors
• Medium 2 speed--1 resistor
When the operator requests High speed, the HVAC control assembly applies ground directly to the blower motor through the high blower motor control circuit. The resistor card has a fusible link which, if blown, will require that the card be replaced.

Its
faulty blower motor resistor. The low and medium speed on blower motor is
controlled by resistor and its actual speed is high. So because resistor is not
working, blower motors actual speed which is high is working.

It
can be faulty blower motor or faulty blower motor resistor. Get the blower
motor inspected. If it's faulty it has to be replaced.

=======

For
blower motor testing and replacing, check the link below: ----

Blower motor Testing/replacing for all vehicles?

Its
faulty blower motor resistor. The low and medium speed on blower motor is
controlled by resistor and its actual speed is high. So because resistor is not
working, blower motors actual speed which is high is working.

It
can be faulty blower motor or faulty blower motor resistor. Get the blower
motor inspected. If it's faulty it has to be replaced.

Its
faulty blower motor resistor. The low and medium speed on blower motor is
controlled by resistor and its actual speed is high. So because resistor is not
working, blower motors actual speed which is high is working.

It
can be faulty blower motor or faulty blower motor resistor. Get the blower
motor inspected. If it's faulty it has to be replaced.

Its
faulty blower motor resistor. The low and medium speed on blower motor is
controlled by resistor and its actual speed is high. So because resistor is not
working, blower motors actual speed which is high is working.

It
can be faulty blower motor or faulty blower motor resistor. Get the blower
motor inspected. If it's faulty it has to be replaced.

Often times the problem is the blower resistor not the switch.You would use a meter or a test light to check for power at the resistor and also the switch. The resistor controls power to the blower motor to adjust speed.

The blower motor resistors are known for going bad. Usually its because the blower starts drawing too much current. What I would recommend doing if you can and have the tools, is to check the blower motor connector to make sure 12 volts is present. If so than I would recommend replacing the blower motor resistor. Then if the same issue ever happens again and the blower motor resistor is bad again, you will need to replace the blower motor and the blower resistor as the final repair.
Here is some info about testing the resistor using a multimeter it you have one:

DIAGNOSIS AND TESTING- BLOWER MOTOR RESISTOR
WARNING:ON VEHICLES EQUIPPED WITH AIRBAGS, DISABLE THE AIRBAG SYSTEM BEFORE ATTEMPTING ANY STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT PANEL COMPONENT DIAGNOSIS OR SERVICE. DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE, THEN WAIT TWO MINUTES FOR THE AIRBAG SYSTEM CAPACITOR TO DISCHARGE BEFORE PERFORMING FURTHER DIAGNOSIS OR SERVICE. FAILURE TO TAKE THE PROPER PRECAUTIONS COULD RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.
Both the front blower motor resistor and the optional rear heater-A/C unit blower motor resistor can be diagnosed using the procedure that follows. For circuit descriptions and diagrams, refer to Air Conditioning/Heater in Group 8W - Wiring Diagrams.

Check for continuity between each of the blower motor switch input terminals of the resistor and the resistor output terminal. In each case there should be continuity. If OK, repair the wire harness circuits between the blower motor switch and the blower motor resistor block or blower motor as required. If not OK, replace the faulty blower motor resistor block.

Here is the information on replacing the resistor:

REMOVAL

Disconnect and isolate the battery negative cable.

From the passenger foot well area, remove the two hex screws used to secure the blower motor resistor.